17.Letter Combinations of a Phone Number

Letter Combinations of a Phone Number Total Accepted: 34954 Total Submissions: 134310  Question   Solution

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

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思路：回溯法。既不是子集树，也不是排列树。普通回溯。

class Solution { public:
    void solve(string &digits,int pos,unordered_map<char,string> &my_map,vector<string> &res,string &one_res){
        if(pos>=digits.size()){
            res.push_back(one_res);
            return;
        }else{
            string tem_string=my_map[digits[pos]];
            for(int i=0;i<tem_string.size();++i){
                one_res+=tem_string[i];
                solve(digits,pos+1,my_map,res,one_res);
                one_res.pop_back();//C++11中的string还是有这个接口的
            }
        }
    }
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if(digits=="") return res;//简单判错
        unordered_map<char,string> my_map={{'2',"abc"},{'3',"def"},{'4',"ghi"},{'5',"jkl"},{'6',"mno"},{'7',"pqrs"},{'8',"tuv"},{'9',"wxyz"}};
        string one_res;
        solve(digits,0,my_map,res,one_res);
        return res;
    }
};